(1) Field of the Invention
The present invention relates to a semiconductor device suitable for use as a semiconductor optical amplifier employed in, for example, an optical communication system.
(2) Description of Related Art
Along with the recent remarkable increase in communication demand, a wavelength division multiplex (WDM) communication system allowing large capacity transmission with one optical fiber by multiplexing a plurality of signal lights of different wavelengths has been developed.
As an optical amplifier used in such an optical communication system, a semiconductor optical amplifier (SOA) which has a wide wavelength band and a small size and is capable of operating with low power consumption is considered to be desirable.
On the other hand, in recent years, the range of application of a large capacity and high speed photonic network has been enlarged also up to metropolitan access networks which is close to the subscribers. A photonic network applied to a metropolitan access network has a flexible network configuration using an optical add/drop multiplexer (OADM), etc.
In an optical amplifier used in such a network, there is a need to allow automatic gain control (AGC) or automatic power control (APC) to cope with any change in the number of wavelengths to be multiplexed or a change in the intensity of light to be inputted.
In order to realize such automatic gain control or automatic power control, there is demanded the gain control of the amplifier so as to obtain a desired gain or a desired output power while monitoring the input/output power. In this case, a power monitor (photodetector or power meter) for monitoring the input/output power is demanded.
However, provision of a power monitor as an individual part connected through an optical fiber would lead to increases in cost.
For this reason, there have been proposals to integrate a photodetector with a semiconductor optical amplifier for the reduction of cost (see, for example, R. G. Broeke et al. “ALL-OPTICAL WAVELENGTH CONVERTER WITH AMONOLITHICALLY INTEGRATED DIGITALLY TUNABLE LASER” European Conference on optical Communication (ECOC) 2002).